Turning tapered threads. I needed a couple of parts threaded 1/4" NPT, which is a tapered pipe thread (1-in-16 taper on diameter, or 1.79° angle to axis). Lacking a die, I had to figure out a way to turn them on the lathe.
The topslide is set to the required angle, and the OD taper turned as you would with any taper. A dial test indicator is then mounted on the lathe bed and touches the edge of the top slide. As the threading cut is made by turning the chuck (in my case, with a manual crank handle), the cross slide is moved out to keep the indicator reading constant.
It sounds tricky, but it's actually pretty easy to keep within 0.02mm or so.
I can't claim to have thought on this first - I remember reading about the trick ages ago, but can't remember where. Normally a rear-mounted taper attachment would be used to perform the same function, but they're rarely found on small lathes.
Tags: Turning tapered threads, lathe, machining, threading, pipe thread, plumbing, workshop, engineering, tips